Electron discharge tube circuits



Feb. 25, 11. J. HAANTJES EIAL 'ELECTRON DISCHARGE TUBE CIRCUITS Filed Jan. 4, 19159 ATTdRNEY GAIN CONTROL BIAS.

Patented Feb. 25, 1941 UNITED STATES PATENT. OFFICE nil-scram: DISCHARGE TUBE CIRCUITS ration of Delaware Application January 4, 1939, Serial No. 249,242 In the Netherlands May 19, 1938 13 Claims.

This invention relates to a circuit arrangement for the amplification and/or frequency-changing of electric oscillations whose gain can be controlled by variation of the mutual conductance of at least one of the tubes and in which use is made of negative feedback.

In the low-frequency amplifier art it is general practice to use negative feedback coupling for the purpose of decreasing the distortions caused by the curvatures of the valve characteristic curves. In circuits for the amplification and/or frequency-changing of high-frequency oscillations difficulties also arise due to the curvature of the valve characteristic curves, and among other things, to the occurrence of the so-called cross modulation in wireless receivers and due to the production of harmonics of the signal to be amplified which, particularly in superheterodyne receivers, lead to the production of harmful beat tones. It has been suggested before to use negative feedback coupling for obviating the said disadvantages in high-frequency amplifying and frequency-changing circuits.

In high-frequency circuit arrangements it is common to control the gain by variation of the mutual conductance of the amplifier and mixing valves. Such a gain control is also used in some cases in low-frequency amplifying circuits. Now,

"' by variation of the mutual conductance of the tubes a negative feedback is arranged in a manner which is common practice in the low-frequency amplifying art by causing a part of the voltage occurring in the anode circuit to occur in phase-opposition in the input circuit, the disadvantage exists that the intensity of the feedback decreases with decreasing mutual conductance.

This not only has the effect of counteracting the desired gain control but in the usual manner of control in which the gain decreases with increasing signal intensity the disadvantage exists that the feedback is a minimum when powerful signals occur in which case the probability of difficulties due to the curvature of the characteristic curves is a maximum, whereas in the event of feeble signals in which case there is only little probability of difficulties consequent upon the curvature of the characteristic curves, a maximum feedback is obtained so that the sensitiveness of the amplifier is unnecessarily decreased.

The invention has for its object to obviate these difficulties. According. to the invention, the feedback voltage or current is for this purpose derived from the circuit or circuits of one or more current carrying electodes of at least one of the if in a circuit in which the gain is controlled contolled tubes whose alternating current increases or at least remains constant if the gain is decreased.

According to the invention, the feedback voltage or current is preferably derived from the circult of a current carrying electrode, arranged between the anode and the input contol grid of a multigrid valve and the gain control is obtained by variation of the bias of a gain control grid I arranged between the said current carrying electrode and the anode. In a suitable embodiment of the invention there is also on the anode side of the gain control grid 8. current carrying electrode which is electrically connected to the firstmentioned current carrying electrode.

According to a further feature of the invention the feedback is brought about by including in the circuit of the said current carrying electrode or electrodes a feedback impedance which is also included in the input circuit of the valve and the anode circuit for the frequency of the oscillations to be amplified and/ or to be changed in frequency is connected to that end of the feedback impedance which is connected to the cathode. In frequency changing circuit arrangements the gain 25 control grid and the anode have preferably arranged between them a control grid to which the locally generated oscillations are fed, a screen grid being arranged on either side of this latter control grid, the circuits of the said control grid and the said screening grids for the frequency of the oscillations to be transformed are connected to that end of the feedback impedance which is connected to the cathode.

According to a further embodiment of the in- 35 vention the circuit of the said current carrying electrode or electrodes includes an inductance coll which is shunted by an ohmic resistance whose value is low compared with the impedance of the inductance and which is coupled inductively to a second inductance which is included in the input circuit of the valve in series with a tuned oscillatory circuit.

A circuit arrangement in which the intensity of the feedback is independent of the gain can be obtained by including in the input circuit of the tube a feedback impedance which has passing through it the alternating current of all current carrying electrodes; the gain control being effected by variation of the bias of a gain control grid which is arranged on the side of the input control grid remote from the cathode and which is separated therefrom by one or more screening grids.

In order that the invention may be clearly 1m.-

derstood and readily carried into effect the same will now be described more fully with reference to the accompanying drawing wherein Fig. 1 shows a high frequency stage which embodies the present invention, Fig. 2 shows a modification of the circuit shown in Fig. 1, Fig. 3 shows a frequency-changing or mixing circuit for use in a superheterodyne type of receiver according to the invention, and Fig. 4 shows a further modification of the circuit of Fig. 1.

Fig. 1 shows the first stage of a radio receiver in which use is made of a multigrid valve I comprising in succession a cathode 2, an input control grid 3, a current carrying electrode 4, a gain control grid 5, a screening grid 6, a suppressor grid I, which is connected to the cathode, and an anode 8. The antenna 9 is connected to earth through an inductance I0 which is coupled inductively to a second inductance II, the latter forming part of an oscillatory circuit which, by means of a variable condenser I2 can be tuned to the frequency of the received oscillations. The oscillatory circuit II, I2 is connected to the input control grid 3 of the valve I. The anode circuit of the valve I includes an inductance I3 coupled to a tuned output circuit I4. The amplified high-frequency oscillations are obtained from the terminals I5 and I6. The anode circuit and the circuit of the screening grid 6 are connected, so far as high frequencies are concerned, to the cathode 2 of the valve I by means of a condenser H. The circuit of the current carrying electrode 4 includes a condenser I9 and a feedback impedance I8, the latter being also included in the input circuit of the valve I. The feedback impedance I8 is shown in the figure as a high-frequency non-shunted ohmic resistance. Instead of using an ohmic resistance, use may, however, be made of another feedback impedance, for example, of a greatly damped parallel resonance circuit. The gain control grid 5 has supplied to it via a conductor 20 a variable bias for gain control, said bias being varied by hand or automatically in accordance with the amplitude of the received signals. So far as high frequencies are concerned the gain control grid is connected to the cathode via a condenser 2| so that the feedback voltage cannot occur in the gain control grid circuit. Since in many cases the conductor 20 is earthed at another point so far as high frequencies are concerned, it is preferable that the conductor 20 should include a filter resistance 22.

The operation of the circuit arrangement described is as follows. The gain control grid 5 is placed at earth potential or at a negative potential so that part of the electrons emitted by the cathode will change their directions in the proximity of this grid and will flow to the positive electrode 4. The more negative the gain control grid 5, the greater will be this part. If the gain of the tube is decreased by an increase of the negative bias of the gain control grid 5, the alternating current flowing to the electrode 4 will consequently increase so that the alternating voltage set up across the feedback impedance I8 increases. The intensity of the feedback therefore increases if the gain is reduced so that in the event of powerful signals, in which case the gain is generally to be limited to a minimum value and there is moreover a great risk of difiiculties occurring due to the curvature of the characteristic curve, the maximum negative feedback occurs, whereas in the event of feeble signals the feedback is low and the sensitiveness of the receiver is consequently not unnecessarily limited.

An important advantage of the circuit arrangement described resides in that the desired gain control is assisted by the variation of the intensity of the feedback so that a highly efiicient gain control is obtained.

If the feedback impedance I8 is formed as an ohmic resistance, the latter may also serve fundamentally for generating the required negative bias for the input control grid 3. For obtaining sulficient feedback the feedback impedance should, however, generally be formed by an ohmic resistance of the order of magnitude of about 2000 to 4000 ohms, said value being too high for generating a suitable bias for the input control grid. In order to obviate this difliculty, the D. C. circuit of the input control grid in the circuit arrangement of Fig. 1 includes only a part of the resistance I8 due to the fact that a point, intermediate the ends of the resistance I8, is connected to the control grid 3 via a resistance 23 whose value is high compared with that of the resistance I8, whereas the end of the resistance I8 which is remote from the cathode is connected to the said control grid via a condenser 24 which forms part of the oscillatory circuit I I, I2. This measure ensures that the total high-frequency voltage is set up across the resistance I8 but only a part of the D. C. voltage occurring across this resistance is fed to the input control grid.

In many commercially obtainable multigrid valves the grids 4 and 6, arranged on either side of the regulating grid 5, are electrically connected within the tube. Such tubes may also be used in the circuit arrangement according to the invention since generally the alternating current of the grid 4, which increases with increasing negative bias of the gain control grid, substantially exceeds the alternating current of the grid 6, which decreases at the same time. The use of such tubes may even be advantageous if the feedback set up in the event of feeble signals is desired not to be limited excessively.

A tube of this kind is used in the circuit arrangement of Fig. 2. In this embodiment of the invention an inductive feedback by means of coils 25 and 26 is substituted for the feedback impedance I8. The common circuit of the grids 4 and 6 includes the coil 25 which is connected to earth via a condenser 21. In order to obtain the correct phase of the feedback voltage the coil 25 is connected in parallel with an ohmic resistance 28 whose value is small compared with the highfrequency impedance of the coil 25 so that the voltage set up across this coil is substantially in phase with the alternating current of the grids 4 and 6. The coil 25 is coupled inductively to the coil 26 which is included in the input circuit of the valve I in series with the oscillatory circuit II, I2.

It must be observed that the circuit arrangement generally in use with positive feedback and in which the feedback coil is coupled to the inductance of the input oscillatory circuit is preferably not used in the present instance since the use of this circuit arrangement for negative feedback would lead to considerable damping of the input oscillatory circuit and to a falling-off in selectivity. Moreover, this circuit would exhibit the additional drawbacks that complete linearising of the valve characteristic curve would not be obtained since for this purpose the harmonics set up by the curvature of the characteristic curve would have to occur also in the feedback voltage and due to the induction of the feedback voltage in the input oscillatory circuit the har- .monics would be weakened with respect to the fundamental frequency- In the circuit arrangement of Fig. 2 it is possible to obtain the desired negative bias of the input control grid in the usual manner by means of a resistance 29 included in the cathode conductor and short-circuited so far as high frequencies are concerned, by means of a condenser 29. a

For the rest the operation of the circuit arrangement of Fig. 2 is identical with that of the circuit arrangement of Fig. 1. Fig. 3 shows a frequency-changing circuit arrangement embodying the invention which comprises a mixing valve 30 having a cathode 3|, an input control grid 32, a current carrying electrode 33, a gain control grid 34, a, screening grid 35, an oscillator control grid 36, a second screening grid 31, a suppressor grid 38 connected tothe cathode, and an anode 39. The received oscillations are fed to the input control grid 32. The anode circuit includes an intermediate-frequency circuit 40 which is coupled inductively to a second intermediate-frequency circuit 4|. The oscillations whose frequency is changed are taken from the terminals [5 and It. A negative feedback is obtained by means of a feedback impedance I8 which is included in the input circuit and has the alternating current of the electrode 33 passing in it, for which purpose this electrode is connected, so far as high frequencies are concerned, via a condenser l9 to that end of the feedback impedance which is remote from the cathode. In order to obtain the desired negative bias for the input control grid 32 the resistance I8 is connected in parallel with the series combination of a high-frequency choke 46 and a resistance 41 so that the total D. C. resistance in the cathode conductor is less than the resistance of the feedback impedance l8, the latter being generally too heavy to ensure a suitable bias for the control grid 32.

The anode circuit and the circuits of the screening grids 35 and 3! are connected, so far as high frequencies are concerned, to that end of the feedback impedance 18 which is connected to the cathode so that the feedback is only governed by the alternating current of the electrode 33.

The gain control is obtained by feeding a variable bias to the gain control grid 34. If the gain control grid is rendered more negative, the alternating current of the anode 39 decreases, whereas the alternating current of the electrode 33 increases. The intensity of the negative feedback consequently decreases with increasing gain.

The local oscillations are preferably fed to the oscillator control grid 33 in such manner that the feedback voltage set up across the impedance l8 does not occur in the circuit of the oscillator control grid. For this purpose, in the present circuit arrangement the local oscillations set up by a separate oscillator valve 42 are transmitted inductively to a coil 43 connected between the oscillator control grid 36 and the cathode 3!, the desired negative bias of the oscillator control grid being obtained by means of a grid condenser 45 and a leak 44. The inductive coupling between the oscillatory circuit 42' of the oscillator 4,2 and the circuit .of the control grid 36 upon which the local oscillations are impressed offers the particular advantage that the tuning condenser of the oscillator can be connected to earth, which in the event of a different manner of coupling, would lead to a short-circuit of the feedback impedance l3.

Due to the fact that the oscillator control grid 35 lies on that side of the gain control grid 34 which is remote from the cathode, and in addition is separated from the gain control grid by a screening grid 35, the oscillator control grid exercises practically no controlling action on the electrons that change their directions in the proximity of the gain control grid so that the alternating current of the electrode 33 contains almost uniquely the frequencies of the received oscillations and does'not contain the frequency of the local oscillations.

For a satisfactory operation of the circuit arrangements so far described it is desirable that the feedback current or voltage, derived from the circuit of the electrode 4 (Fig. 1) or 33 (Fig. 3) respectively has practically the same time variations as the cathode alternating current. For a given Value of the control voltage there must therefore be between the instantaneous values of the cathode alternating current and the alternating current of the electrode 4 or 33 respectively, preferably a practically constant ratio, or v,

in other words the ratio in which the stream of electrons which passes through the apertures of the input control grid 3 or 32 respectively is divided by means of the regulating grid 5 or 34 respectively between the electrode 4 or 33 respectively and the other conducting electrode of the valve l or 33 respectively must be practically independent of the intensity of the said stream of electrons and hence of the voltage of the input control grid 3 or 32 respectively.

This condition can be satisfied by a special valve construction and/or by a special choice of the biases of the various electrodes, for example by taking care that only a very low space charge can be formed in the proximity of the control grid 5 of 34 respectively.

A further method of rendering the current distribution, brought about by the gain control grid 5 or 34 respectively, practically independent of the voltage of the input control grid 3 or 32 respectively consists in feeding a suitably chosen part of the input alternating voltage to the gain control grid, which may be effected in a simple manner by connecting the gain control grid so far as high frequencies are concerned, to a suitably chosen point of the input oscillatory circuit II, l2 instead of connecting it to the oathode via the condenser 2|. If it is assumed that the influence of the voltage, set up across the input control grid, on thecurrent distribution, brought about by the gain control grid, is due to the controlling action of the input control grid on the space charge set up in the proximity of the regulating grid, the effect of the above measure may be due to the fact that part of the input alternating voltage which is supplied to the gain control grid neutralises the action of this space charge. It is found that this compensation can be obtained for all values of the gain control voltage by feeding the same part of the input alternating voltage to the gain control grid. The action aimed at in this case is independent of the value of the gain control voltage. In the circuit arrangement shown in Fig. 3 it is in addition desirable that the current distribution brought about by the oscillator control grid 36 is independent as far as possible of both the voltage of the input control grid 32 and the voltage of the gain control grid 34.

All thecircuit arrangements described so far exhibit the property that the intensity of the feedback decreases with increasing gain, said property being generally advantageous since it has the effect of ensuring a very efficient gain control, while at the same time the sensitiveness of the circuit arrangement to feeble signals is high. There may, however, also be cases in which such efficient gain control or such high sensitiveness of the circuit arrangement respectively is not desirable and in which consequently a negative feedback may be used whose intensity is independent of the gain.

A circuit arrangement which permits of obtaining a negative feedback substantially independent of the gain is shown in Fig. 4. This circuit arrangement is substantially identical with that of Fig. 1 but is differentiated therefrom by the fact that the anode circuit instead of being connected, so far as high frequencies are concerned, to the cathode is connected via the condenser [9 to that end of the feedback impedance [8 which is remote from the cathode so that the feedback impedance has passing through it the alternating current of all conducting elec trodes of the tube I. Since variation of the bias of the gain control grid 5 primarily results only in an alteration of the current distribution between the electrodes 4, 6 and 8 but not in an alteration of the emission stream, the total current of all conducting electrodes is substantially independent of the gain control voltage. The feedback voltage set up across the impedance [8 will consequently also be practically independent of the gain.

A particular advantage of the circuit arrangement of Fig, 4 resides in that it can be arranged in a most simple manner for changing the frequency of the received oscillations. For this purpose it is only necessary to connect between the gain control grid 5 and the point of connection of the condenser 2| to the resistance 22 an impedance across which are set up the local oscillations, for example the inductance 43 shown in Fig. 3. Thus a frequency-changing arrangement with negative feedback is obtained which is substantially simpler than the circuit arrangement shown in Fig. 3 but which has the disadvantage compared with the latter circuit that the gain control is less efficient and the sensitiveness' to feeble signals is also less.

For obtaining the desired bias for the input control grid 3 use is made in the circuit arrangement shown in Fig. 4 of a high-frequency choke 48 which short-circuits'for direct current a part of the resistance l8.

Although all the embodiments described have reference to the input stage of a wireless receiver the invention can also be used at one or more following stages. The invention is also applicable to low-frequency amplifier circuits whose gain is controlled by variation of the mutual conductance of the amplifier valves.

We claim:

1. A circuit comprising an electron discharge tube provided with a cathode, an anode and a plurality of grid electrodes interposed between the cathode and anode, a signal input circuit connected to one of the grid electrodes, an output circuit connected to the anode, means for impressing a feedback voltage on the input circuit in phase opposition to the signal voltage, and

means for applying a gain control voltage to one of the grid electrodes other than the signal grid, the intensity of the negative feedback varying directly with changes in the numerical value of the gain control voltage.

2. A circuit comprising an electron discharge tube provided with a cathode, an anode and a plurality of grid electrodes interposed between the cathode and anode, a signal input circuit connected to one of the grid electrodes, an output circuit connected to the anode, means for impressing a feedback voltage on the input circuit in phase. opposition to the signal voltage, said feedback means comprising an impedance con nected to the cathode and included in a path common to the input circuit and the circuit of one of the grid electrodes, and means for applying a gain control voltage to one of the grid electrodes other than the signal grid.

3. A circuit comprising an electron discharge tube provided with a cathode, an anode and a plurality of grid electrodes interposed between the cathode and anode, a signal input circuit connected to one of the grid electrodes, an output circuit connected to the anode, means for impressing a feedback voltage on the input circuit in phase opposition to the signal voltage, said feedback means comprising an unbypassed resistance connected to the cathode and included in a path common to the input circuit and the circuit of one of the grid electrodes, and means for applying a gain control voltage to one of the grid electrodes other than the signal grid.

4. A circuit according to the invention defined in claim 3 wherein a direct current connection is provided between the signal control grid and the unbypassed feedback resistance for supplying a direct current bias for the signal control grid.

5. A circuit according to the invention defined in claim 3 wherein an intermediate point on the unbypassed feedback resistance is connected to the signal control grid for impressing thereon a suitable negative bias.

6. A circuit according to the invention defined in claim 3 wherein an impedance is connected in shunt to the feedback resistance whereby the total direct current resistance in the cathode circuit is less than the feedback resistance and of suflicient value to provide negative bias to the control grid.

7. A circuit comprising an electron discharge tube provided with a cathode, an anode and first, second and third grid electrodes interposed between the cathode and anode, a signal input circuit connected to the first grid electrode, means for impressing a feedback voltage on the input circuit in phase opposition to the signal voltage, said feedback means comprising an impedance connected to the cathode and included in a path common to the input circuit and the circuit of the second grid electrode, means for applying a gain control voltage to the third grid electrode, the intensity of the negative feedback varying directly with changes in the numerical value of the gain control voltage, and an output circuit connected to the anode.

8. A circuit according to claim 7, wherein a screen grid is interposed between the gain control grid and the anode, and a bypass condenser is connected between said screen grid and the cathode side of the feedback impedance.

9. In a superheterodyne receiver, a frequency changing circuit comprising an electron discharge tube provided with at least a cathode, an anode, first, second, third and fourth grid electrodes interposed between cathode and anode, a signal input circuit connected to the first grid, means included in the circuit of the second grid for impressing a feedback voltage on the input circuit in phase opposition to the signal voltage, means for applying a gain control voltage to the third grid electrode, means for impressing local oscillations on the fourth grid electrode, and means included in the anode circuit for deriving an intermediate frequency resulting from the interaction between the signal oscillations and the local oscillations.

10. In a superheterodyne receiver, a frequency changing circuit comprising an electron discharge tube provided with a cathode, an anode and a plurality of grids interposed between said cathode and anode, said grids comprising a signal control grid, a current carrying grid, a gain control grid and a local oscillator grid disposed in the order named, a signal input circuit connected to the signal control grid, means included in the circuit of the current carrying grid for impressing a feedback voltage on the input circuit in phase opposition to the signal voltage, said feedback meanscomprising an impedance connected to the cathode and included in a path common to the input circuit and the circuit of the current carrying grid, means for applying a gain control voltage to the gain control grid, means for impressing local oscillations on the oscillator grid, and means included in the anode circuit for deriving an intermediate frequency resulting from the interaction between the signal oscillations and the local oscillations.

11. A circuit according to claim 10 wherein the local oscillator grid is shielded on both sides by a screen grid, and a bypass condenser connected between said screen grid and the cathode side of the feedback impedance.

12. A circuit according to claim 10 wherein the feedback impedance is constituted by an unbypassed resistance.

13. A circuit according to claim 10 wherein the feedback impedance is constituted by a resistance, and a series connected resistance and choke coil are connected in shunt to the feedback resistance for reducing the total direct current resistance in the cathode circuit to a suitable value for providing negative bias for the signal control grid.

J OHAN HAANTJES. 'BERNARDUS DOMINICUS HUBER'IUS TELLEGEN. 

